Are Propellers Fin-ished?

Published: 07/05/2010

If you've ever been to an aquarium or a zoo, you've probably admired the feisty penguins. They can squiggle through water faster than 10 miles per hour, turn on a dime, and leap onto shore, all in one smooth movement.
Dolphins and seals can perform similar aquabatics.
These marine animals are more than just fun to watch. They're also inspiring engineers to look for better ways of propelling boats. You never see a submarine do what a penguin can do, but wouldn't it be cool if it could?
Propellers let ships travel in a relatively straight line over great distances. Today's engineers are trying to design vessels that can do a lot more than that. They want boats able to withstand stormy conditions that would shatter an existing craft. They want boats that can maneuver quickly in tight spaces. They want boats that can sense currents or waves and respond in a split second to hold their position. In effect, they want to reinvent the penguin—or perhaps the whale or fish.
A penguin's flippers are a good starting point.
All dressed up
Propeller blades just spin. Penguin flippers do much more.
A penguin's flipper is like a hard, stiff paddle covered with tiny feathers. It's shaped a bit like an airplane wing. A flipper can flap up and down, move forward and backward, and twist around at the joint where it's attached to the penguin's body.
At the Massachusetts Institute of Technology, researchers are working on a new propulsion system for ships that mimics a penguin's flippers. Their artificial wooden flippers move a boat forward or backward by generating high-energy rings of spinning water. Other flipper movements steer the craft right, left, up, or down.
The MIT team is now testing how various flipper movements affect a boat's motion, doing experiments in giant basins of water.
The scientists envision using a pair of flippers in place of a propeller to move a boat along. More futuristic vessels could have as many as 50 flapping flippers, each one moving independently.
But it'll take many more years of research before the Navy or anyone else can launch high-tech ships driven by flippers.
A new kind of fin
Flexibility also helps move things along in water.
Marine animals such as dolphins and seals aren't made of stiff materials. They're squishy, like human skin and muscle.
Flexible materials can store energy in ways that stiff ones can't. When a dolphin flexes its tail as far as it'll bend, it stores energy in its body—just like a stretched rubber band. When the tail slams down and straightens, this stored energy is released, and the dolphin shoots forward.
Engineers at Nekton Research, a company in Durham, N.C., have designed flexible fins for an underwater vehicle to take advantage of such cycles of storing and releasing energy.
The craft, called PilotFish, is shaped like a giant egg with four fins coming out of its waist. It's more than 3 feet long and weighs 350 pounds.
PilotFish can't travel long distances quickly. Instead, maneuverability is its specialty. And it can get going in a fraction of a second. Moreover, unlike any other watercraft, it can stop almost instantaneously just by slamming its fins forward.
"The thing looks like it hit a wall. It stops dead," says Chuck Pell, who helped design the fins. "The only other things that can do that are alive."
PilotFish is designed to operate in water too turbulent for other craft. For example, it could be used to inspect the underwater portions of structures such as bridges and docks.
A river's waves or current can easily overcome or carry away a propeller-driven craft before it can perform an inspection. In contrast, PilotFish reacts to its environment quickly enough to stay in place. If the craft encounters an unexpected object, it can immediately stop to avoid bumping into it. If a wave rolls it over, PilotFish can right itself before the next wave comes.
To accomplish all this, PilotFish's fins generate huge forces. "You have to careful around it. You could break an arm," Pell says. He notes that he once ended up with a sprained wrist when a moving fin accidentally struck his hand.
Whale watch
For their size, humpback whales are surprisingly agile. This 50-foot, 30-ton animal can swim in a tight corkscrew pattern, sometimes less than 10 feet across.
The whales do this not for fun but to capture a meal. They blow bubbles as they swim in this spiral pattern, creating a rising barrier around a cylinder of water. Tiny shrimp and small fish get trapped in the cylinder, and the whale simply swims up through the concentrated feast for its meal.
Scientists have long wondered how humpbacks manage this feat. They've been particularly curious about bumps along the leading edge of a humpback's long, narrow flippers.
To find out, researchers built two artificial whale flippers. One flipper had a scalloped edge, and the other was smooth. They then tested the two flippers in a wind tunnel. Although air is much less dense than water, it's still a fluid, and the researchers could adjust the air's speed so that it behaved like water rushing over a humpback's flipper.
The scientists found that the bumps reduce drag and increase a flipper's lift so it behaves more like an airplane wing. This extra lift and reduced drag lets a humpback whale make sharper turns than other whales can make.
Someday, engineers designing flippers or fins to drive boats and submarines might add bumps or scallops, too.
Kayak flippers
Artificial fins inspired by one marine animal, the penguin, are already available—though not where scientists might have predicted. They're in a foot-powered propulsion system for kayaks designed by engineers at Hobie Cat in Oceanside, Calif.
Instead of paddling, you sit in the kayak and pedal with your feet. Your pedaling powers two flexible fins.
At the start of each stroke, the fins twist and flex in such a way that they assume the shape of a propeller blade. A penguin's flipper flexes in the same way when the swimming bird wants to move itself forward.
The fins move larger volumes of water than a traditional oar can yet require less energy to do so. This lets kayakers go farther and faster, without getting as tired as when paddling with oars.
Hobie Cat's pedaled kayaks are leading the way in applications of nature-inspired flipper design. Other applications are bound to follow.
Maybe someday you'll be able to go to an aquarium show featuring underwater vehicles, gliding gracefully, racing around rocks, and leaping out of the water to wow a crowd—doing what comes naturally to penguins and dolphins.